Learning
MESUREMENT OF SURFACE TENSION
It is well known that liquid can exist in a liquid state because of the attractive forces between molecules called Intermolecular Forces (van del Waals' forces).
The molecules in bulk are surrounded by other molecules and are attracted in every direction with equal force. Molecules exposed to the surface however, are unstable because the attractive forces are not equal, and they are drawn into the bulk.
As a result, the liquid voluntarily tends to contract the surface area, and will equilibrate when the surface area reaches a minimum. From everyday life, we know that small droplets and bubbles are sphere-shaped. This shape can be explained by the phenomenon that surface area becomes minimum when the droplet forms a sphere if the liquid volume is the same. The intermolecular forces to contract the surface is called Surface Tension.
TYPICAL MESUREMENT METHODS
There are several ways to measure Surface Tension. Here are a few of the most common:
Wilhelmy Plate Method
A detection plate made of platinum or glass is used in this method. When the bottom of a vertically-oriented detection plate makes contact with a liquid surface, the liquid wets the plate surface upward and meniscus is created. At this moment, the surface area of the liquid is expanded and surface tension tends to contract the surface area as a counteraction, and immerse the plate downward.
This method determines surface tension by measuring the force bringing the plate downward via a counter balance. Surface Tension is a force for each length, and is calculated in relation to the perimeter of plate (double the plate's thickness and width) corrected for buoyancy (shρg as in the figure) caused by immersing the plate.
The unit for Surface Tension used to be “dyne/cm”, but has been standardized to “mN/m” as a SI unit.
Pendant Drop Method
The droplet hung on the needle tip forms a pendant shape, not a sphere. In this state, the equation above is given in thermodynamics. This is called the “de/ds method”.
This method determines Surface Tension by image analysis of the pendant’s profile and the density of the droplet.
The fundamentals of the profile of droplets is described by the Young-Laplace equation, and the correction factor of the above de/ds method is calculated from the Young-Laplace equation.
To execute the measurement based on the Young-Laplace theory, a process to determine a theoretical fitting curve by plotting a number of dots on the pendant profile is required. Compared with the de/ds method which determines Surface Tension by only 5 points on the profile, the results by Young-Laplace are more accurate because it reaches a measurement from a few hundred points,. Young-Laplace also requires much faster computing processing, but thanks to advances in personal computing, it is now possible with ease in short time.
As the name suggests, a ring, made of platinum, is used in this measurement. The method was established for measurement of surface tension earlier than the Wilhelmy Plate Method, and has been adopted by some industrial standards like JIS, and ASTM..
The principle of this method is similar to that of Wilhelmy Plate, and the force affected on the detection component (the ring) is measured by the counter balance unit. However, this method requires lowering the liquid surface to detect the peak force upon removal of the ring from the liquid surface, while the Wilhelmy Plate method keeps the plate bottom immersed in the liquid surface.
The Wilhelmy Plate method has surpassed the du Nouy Ring method in popularity, due to the following disadvantages:
- The thin wire ring material changes shape even from gentle impact, which affects the measurement results.
- The ring is less accurate with higher viscosity materials
- Variations in surface tension over time cannot be measured due to the mechanics of removing the ring from the surface.
